<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <div style="border: 1px solid red;
                width: 100px;
                height: 100px;">
        <div style="background-color: green;
                    width: 30px;
                    height: 30px;
                    margin-left: 10px;
                    margin-top: 10px;">
        </div>
        <!--margin的意思是外面最大的标签和内部标签的框的边距，内部标签的位置可以往下超过父标签的边距 -->
    </div>
</body>
</html>